Instrument and Telescope Summary

RATIR is a multi-channel imager for the OAN/SPM 1.5-meter Johnson telescope. This page summarizes RATIR and the telescope for the benefit of potential users.

Status

The two optical channels of RATIR (the r and i channels) were mounted on the telescope on 2012 February 18 and are currently undergoing commissioning. The two infrared channels (the ZY and JH channels) are in transit between GSFC and the observatory.

We will update this page as information on the performance of the instrument and telescope becomes available during testing and commissioning.

Known issues with the instrument and telescope are:

• Reduced sensitivity in i;
• Displacements between the fields of the r and i channels; and
• Strong ghosts in the r and i filters (but not in bluer filters).

These issues are described below. We expect to resolve these issues during the summer of 2012.

Architecture

The architecture of RATIR is shown in Figure 1. Three dichroics are used to image the same field with four channels. The dichroics reflect shortwards of 0.69, 0.83, and 1.03 µm and transmit longwards of these wavelengths.

Figure 1. The architecture of RATIR, showing how three dichroics divide light between four channels.

Table 1. Channels

Channel	Detector	Field Size	Pixel Size	Filters
		(arcmin)	(arcsec)
r	CCD	5.4	0.32	SDSS ugr and seven others
i	CCD	5.4	0.32	Fixed SDSS i
ZY	H2RG	10	0.3	Fixed WFCAM Z and Y
JH	H2RG	10	0.3	Fixed MKO J and H

The four channels are summarized in Table 1. In more detail:

• The r channel. This uses a Fairchild 3041 CCD with a UV coating. The detector format is 2048 × 2048 pixels each 15 µm square, but the CCD will normally by binned 2 × 2 to give 0.32 arcsec pixels. The total field of view will be 5.4 arcmin square. This CCD will be equipped with a filter wheel containing up to ten 50 mm diameter filters.

• The i channel. This uses another Fairchild 3041 CCD, but this time with a broad-band coating rather than a UV-optimized coating. The scale and field are close to that of the r channel. This CCD is equipped with a fixed SDSS i filter.

• The ZY channel. This uses a HAWAII-2RG detector with a 1.7 µm cut-off. The detector format is 2048 × 2048 pixels each 18 µm square. Powered optics give a pixel scale of about 0.30 arcsec and a field of about 10 arcmin. A fixed filter is installed close to the focal plane. The filter is split along a N-S axis, with the TBD half of the detector being imaged in a WFCAM Z filter and the TBD half in WFCAM Y filter. The region TBD arcsec wide centered on the join is not imaged cleanly.

• The JH channel. This uses another HAWAII-2RG detector with a 2.5 µm cut-off behind the same powered optics as the ZY channel. Again, a split filter close to the focal plane images the TBD half of the detector in MKO J and the TBD half in MKO H.

Fields

Figure 2 shows the instantaneous fields of the detectors. The effective field of view will depend on the dithering strategy:

• If one wishes to obtain images in riZYJH, one will need to dither between the two regions common to riZJ and riYH. If this is done, the effective field of view will normally be slightly smaller than 2.7 × 5.4 arcmin.

• If one wishes to obtain images in ZYJH, ignoring the CCDs, one will need to dither between the two regions common to ZJ and YH. If this is done, the effective field of view will normally be slightly smaller than 5 × 10 arcmin.

• If one wishes to obtain images only with the CCDs, the effective field of view will be 5.4 × 5.4 arcmin.

Of course, it will be possible to map larger areas by mosaicing multiple fields.

Figure 2. The approximate fields over the RATIR detectors, showing the regions imaged in riZJ, riYH, ZJ, and YH. Any of the filters in the filter wheel can substitute for the r filter. The other filters are fixed. Whether ZJ or YH is to the east is TBD.

Issue: During commissioning we discovered that the fields r and i channels are not coincident. The i channel observes a field that is about 50 arcsec north of the r channel. We are investigating this offset.

Filters

RATIR includes fixed iZYJH filters. We intend that the SDSS ugr filters be always installed in the filter wheel in the r channel. Table 2 shows the approximate bandpasses of the ugriZYJH filters.

Table 2. Approximate bandpasses of the ugriZYJH filters.

Name	Bandpass	<λ>	Δλ	<λ>/Δλ
	(µm)	(µm)	(µm)
u	0.328-0.386	0.357	0.058	6.2
g	0.407-0.533	0.470	0.126	3.7
r	0.560-0.675	0.618	0.145	5.4
i	0.700-0.820	0.760	0.125	6.3
Z	0.830-0.925	0.878	0.095	9.2
Y	0.970-1.070	1.020	0.100	10.2
J	1.170-1.330	1.250	0.160	7.8
H	1.490-1.780	1.635	0.290	5.6

The CATT will decide which additional filters will be installed in the other seven slots in the filter wheel, from possibilities that include:

• Johnson UBV
• Strömgren-Crawford uvbyNW
• Nebular filters such as H-alpha.

At the moment, the following filters are installed:

• SDSS gr;
• Johnson BV;
• Strömgren uvby;
• Thuan-Gunn i (Gi); and
• H-alpha.

Issue: The SDSS u filter is not currently installed because the filter delivered by the vendor does not meet the specification for the bandpass. We are in the process of replacing the filter.

Telescope

The telescope is located at

longitude L = -115° 28' 00'',
latitude φ = +31° 02' 43'',

and

height above mean sea level = 2790 meters.

The telescope will initially be able to point between +56° and -33° in declination, ±5h20m in hour angle, and up to 85° in zenith distance.

The standard pointing precision of the telescope is about 15 arcsec RMS. In fields with a sufficient density of moderately bright stars (TBD), a special pointing mode is expected to achieve a precision of 1 arcsec (TBC).

The offset precision is about 2 arcsec RMS. Offsets of up to a few arcmin take about 10 seconds.

The median image quality of the telescope in riZYJH is about 1.0 arcsec.

Operation

The instrument and telescope will be operated robotically.

Observations will be organized into “visits”, consisting of a single slew followed by a number of exposures and offsets. The scheduler will allow the execution of a visit to be constrained by the sky brightness, the airmass, the UTC, and also the likelihood of photometric conditions.

The instrument is expected to be used in two main modes:

• “Infrared Mode”. In this mode, all four instrument detectors are used for science exposures with the same exposure time of up to 60 seconds. The finders are used for guiding.

• “Optical Mode”. In this mode, one of the instrument CCDs is used for a science exposure of up to 1800 seconds and the other is used for guiding.

Sensitivity

The sensitivity of RATIR is given in limiting AB magnitudes. To convert AB magnitudes to approximate Vega-based magnitudes, see Table 7 of Hewett et al. (2006, MNRAS, 367, 454).

The estimated point-source 10-sigma limiting magnitudes in 60 seconds are shown in Table 3. These limits do not include any penalty for sky subtraction and assume image quality of 1.0 arcsec FWHM. We envisage that most infrared observations will be carried out with 60 second exposures, as this exposure time is needed to reach the background limit in griZYJH.

Table 3: The estimated point-source 10-sigma limiting AB magnitudes in dark and bright time in 60 seconds.

Filter	Dark	Bright
u	21.1	20.5
g	22.2	21.2
r	21.8	21.2
i	21.2	20.7
Z	20.1	19.9
Y	19.7	19.7
J	19.9	19.9
H	19.1	19.1

The estimated point-source 10-sigma limiting magnitudes in 960 seconds of infrared mode observations are shown in Table 4 and Figure 3. The 960 seconds of total exposure consists of 16 exposures each of 60 seconds, with 8 exposures on the object in riZJ and 8 exposures on the object in riYH. These limits include the penalty for sky subtraction assuming that 7 sky exposures contribute to the mean sky image and assume image quality of 1.0 arcsec FWHM.

Table 4. The estimated point-source 10-sigma limiting AB magnitudes in dark and bright time in 960 seconds of simultaneous observations in riZYJH in infrared mode.

Filter	Dark	Bright
r	23.3	22.7
i	22.7	22.2
Z	21.2	20.9
Y	20.8	20.8
J	20.9	20.9
H	20.1	20.1

Figure 3. The estimated point-source 10-sigma limiting AB magnitudes in dark and bright time in 960 seconds for simultaneous observations in riZYJH (“infrared mode”).

The estimated point-source 10-sigma limiting magnitudes in 1800 seconds of optical mode observations are shown in Table 5 and Figure 4. These limits do not include any penalty for sky subtraction and assume image quality of 1.0 arcsec FWHM. Since only one CCD will be used for science, only one filter can be observed per exposure, unlike infrared mode observations.

Table 5: The estimated point-source 10-sigma limiting AB magnitudes in dark and bright time in 1800 seconds of observations in optical mode.

Filter	Dark	Bright
u	23.8	22.6
g	24.4	23.1
r	23.8	23.1
i	23.2	22.6

Figure 4. The estimated point-source 10-sigma limiting AB magnitudes in dark and bright time in 1800 seconds for observations in ugri using one CCD (“optical mode”).

Issue: During commissioning we discovered that the AR coating on the lower surface of the first dichroic has a transmission of only about 25% in the i band, which clearly does not meet our specification of better than 1%. This will reduce the sensitivity in the i channel by about 1 magnitude. It does not reduce the sensitivity in the r channel. We are working with the vendor to replace this dichroic, but currently cannot estimate when this will occur.

Ghosts

Issue: The i channel shows strong ghosts. The r channel shows strong ghosts in r, but not in bluer filters. The ghosts have strengths of about 0.5% and are moderately out of focus, with diameters of about 10 arcsec. Tne ghost in r appears about 2 arcmin to the north of the primary image. The two ghosts in i appear about 2 arcmin to the north and 2 arcmin to the south of the primary image. These strong ghosts are the result of the delivered r and i filters being wider than specified and the poor AR coating on the first dichroic. We are working with the vendor to replace these components, but currently cannot estimate when this will occur.